1. Field of the Invention
The present invention relates to a constant velocity universal joint for use with a propeller shaft that is incorporated into four-wheel drive (4WD) vehicles or front-engine rear-drive (FR) vehicles to transmit a rotational drive force.
2. Description of the Related Art
For example, since an FR vehicle is equipped with an engine, a clutch, and a transmission at the front, and with a differential gear and a drive axle at the rear, respectively, it is common to use a propeller shaft for power transmission therebetween. For an FR-based 4WD vehicle, it is also necessary to equip the vehicle with a rear propeller shaft 6 and a front propeller shaft 3, as shown in FIG. 12. These propeller shafts are equipped with a constant velocity universal joint to accommodate variations in length and angle caused by changes in relative position between the transmission and the differential gears. The propeller shafts are of a double-joint or triple-joint type depending on the structure and required characteristics of the vehicle.
From the viewpoint of reducing the weight of the entire vehicle, the propeller shaft incorporates a sliding constant velocity universal joint, referred to as a Lobro-type (or cross groove type), which is light in weight and has a good rotational balance and vibration properties. This constant velocity universal joint is constructed to accommodate an axial displacement, caused by an axial impact resulting from a collision, between the transmission and the differential gears. On the other hand, the propeller shaft incorporates a double offset type constant velocity universal joint (DOJ) or a Rzeppa-type constant velocity universal joint (BJ) other than the aforementioned Lobro-type constant velocity universal joint (LJ).
As shown in FIG. 13, a Lobro-type constant velocity universal joint 12 that forms the propeller shaft has main components such as an inner ring 13, an outer ring 14, balls 15, and a cage 16.
The inner ring 13 has a plurality of track grooves 17 formed on the outer peripheral surface thereof. A reduced diameter portion of a stub shaft 19 fits into a hole formed at the center of the inner ring 13, and the serrations formed on the inner peripheral surface of the inner ring 13 mate with those formed on the outer peripheral surface of the reduced diameter portion of the stub shaft 19, thereby enabling torque transmission. Furthermore, a snap ring 20 installed in an annular groove of the stub shaft 19 causes the stub shaft 19 to be fixedly positioned to the inner ring 13 in the axial direction.
The outer ring 14, disposed on the outer periphery of the inner ring 13, is provided on the inner peripheral surface thereof with the same number of track grooves 18 as that of the track grooves 17 of the inner ring 13. The track grooves 17 of the inner ring 13 and the track grooves 18 of the outer ring 14 form angles opposite to each other with respect to the axial line. The ball 15 is incorporated into an intersecting portion between the track groove 17 of the inner ring 13 and the track groove 18 of the outer ring 14, which are paired with each other. The cage 16 is interposed between the inner ring 13 and the outer ring 14 to hold the ball 15 inside a pocket of the cage 16. The outer ring 14 is coupled to a companion flange 22, having a hollow portion 21, with bolts 24, while an end cap 23 is being sandwiched therebetween. The end cap 23 serves to prevent leakage of grease filled in the constant velocity universal joint 12 as well as intrusion of foreign material.
There is also interposed a sealing device between the outer ring 14 and the stub shaft 19. The sealing device comprises a boot 25 and a metallic boot adapter 26. The boot 25, having a reduced-diameter end portion and an enlarged-diameter end portion, is folded over at the middle in the shape of a letter V in cross-section. The boot adapter 26, cylindrical in shape, has at one end a flange that fits over the outer peripheral surface of the outer ring 14, being fixed to the outer ring 14 with the bolts 24 in conjunction with the companion flange 22 and the end cap 23. The reduced-diameter end portion of the boot 25 is attached to the stub shaft 19 and fastened with a boot belt 27. The enlarged-diameter end portion of the boot 25 is supported by a caulked end portion of the boot adapter 26.
The companion flange 22 is provided with a hole that communicates with the hollow portion 21, and the reduced diameter portion of a stub shaft 28, different from the stub shaft 19, is inserted into this hole to mate therewith by serrations and fixedly fastened with a bolt 29. A ball bearing 30 is press fitted over the reduced diameter portion of the stub shaft 28. The ball bearing 30 is mounted to the vehicle body via a mounting member 31 and supports the stub shaft 28 in a rotatable manner.
The enlarged diameter portion that extends integrally from the reduced diameter portion of the stub shaft 19 closer to the constant velocity universal joint and the enlarged diameter portion that extends integrally from the reduced diameter portion of the stub shaft 28 closer to the companion flange are pressure coupled by friction to one end of tubes 32, 33 at their respective end portions. The other end of the tube 32 is coupled to the transmission via a constant velocity universal joint or the like and the other end of the tube 33 is coupled to the differential gear via a constant velocity universal joint or the like, thereby forming a propeller shaft 11 of a triple-joint type.
The steps shown in FIGS. 14 to 20 are followed to assemble the propeller shaft 11. First, the enlarged diameter portion of the stub shaft 19 closer to the constant velocity universal joint is pressure coupled by friction to the tube 32 (see FIG. 14). Thereafter, the boot belt 27, the boot 25, and the boot adapter 26 are inserted over the stub shaft 19 (see FIG. 15). Then, the stub shaft 19 is press fitted by serrations into the inner ring 13 of an assembly 34 that has pre-incorporated the inner ring 13, the outer ring 14, the balls 15, and the cage 16, being then fixed with the snap ring 20 (see FIG. 16). Subsequently, after grease has been sealed in the boot 25, the boot adapter 26 is press fitted into the outer ring 14 of the assembly 34. The reduced-diameter end portion of the boot 25 is then placed in a groove of the stub shaft 19 and then fixedly crimped with the boot belt 27. Thereafter, grease is sealed in the end cap 23, which is in turn press fitted into the outer ring 14 (see FIG. 17).
Then, the enlarged diameter portion of the stub shaft 28 closer to the companion flange is pressure coupled by friction to the tube 33 (see FIG. 18). The ball bearing 30 is then press fitted over the reduced diameter portion of the stub shaft 28 (see FIG. 19). Then, after having been mated by serrations with the reduced diameter portion, the companion flange 22 is fixedly coupled thereto with the bolt 29 (see FIG. 20). Thereafter, the constant velocity universal joint 12 that has incorporated the aforementioned stub shaft 19 (see FIG. 17) is coupled to the companion flange 22 that has incorporated the stub shaft 28 (see FIG. 20), and then fixedly coupled thereto with the bolts 24 (see FIG. 13).
Now, consider the aforementioned propeller shaft 11 which is constructed to couple the companion flange 22 to the outer ring 14 of the constant velocity universal joint 12 with the bolts 24. This construction makes it necessary to provide the outer ring; 14 with holes for bolts to be inserted therethrough, thereby resulting in an increase in outer diameter and weight of the outer ring 14. Such an increase in Outer diameter of the propeller shaft 11 would be readily restricted due to its interference with other surrounding parts in the vehicle where the propeller shaft 11 is mounted. On the other hand, an increase in weight of the propeller shaft would also interfere with high speed rotation of the propeller shaft.
Furthermore, the propeller shaft 11 requires the bolts 24 for connecting between the companion flange 22 and the outer ring 14 as well as the bolt 29 for connecting between the companion flange 22 and the stub shaft 28, and the two stub shafts 19, 28 since the propeller shaft 11 is configured to connect between the companion flange 22 and the stub shaft 28 with the bolt 29. This results in an increase in number of parts required. An increase in number of parts would result in an increase in number of connections between the parts, thereby causing a degradation of rotational balance in the propeller shaft that rotates at high speeds.
Still furthermore, the assembly process of the propeller shaft 11 requires the additional steps of connecting between the companion flange 22 and the outer ring 14 of the constant velocity universal joint 12, and between the companion flange 22 and the stub shaft 28. This causes an increase in man-hours required for assembly in addition to an increase in number of parts, thereby resulting in an increase in costs of the vehicle.
FIG. 21 illustrates a non-floating Lobro-type constant velocity universal joint configured such that the minimum inner diameter of the cage 16 is greater than the maximum outer diameter of the inner ring 13. The assembly process of the constant velocity universal joint 12 requires the stub shaft 19 to be press fitted into the hole of the inner ring in the assembly comprising the inner ring 13, the outer ring 14, the balls 15, and the cage 16, with the inner ring 13 being prevented from moving axially by means of a support jig or the like. This is because it is necessary to prevent an excessive force from being applied between the balls 15 and the cage 16 since the stub shaft 19 is press fitted into the inner ring 13 before incorporated into the companion flange 22.
Furthermore, some Lobro-type constant velocity universal joint is configured such that the track grooves 17, 18 are elliptical or Gothic arched in shape, and in angular contact with the ball at a contact curvature ratio of 1.02 to 1.05 to the ball curvature and at a contact angle of 35 to 45 degree. This provides a slight vertex clearance VC at the bottom of the track groove of the inner and outer rings 13, 14 with the balls 15 sitting therein. In addition, this constant velocity universal joint is designed such that the balls 15 are controlled in the intersecting portions between the track grooves 17, 18 of the inner and outer rings 13, 14, and enabled to easily rotate in the axial direction, with a clearance between the track grooves of the inner and outer rings and the balls (i.e., a PCD clearance) being employed as a negative clearance, or with a pre-load being provided for the balls. Thus, the constant velocity universal joint is mainly used for a propeller shaft that is required to provide for a good rotational performance at high speeds.
In such a Lobro-type constant velocity universal joint that provides a pre-load for the ball, the amount of increase in temperature of the universal joint (T) is correlated with the number of rotations (N) and the operating angle (θ). In other words, the universal joint is characterized in that the temperature rises as the value of the number of rotations (N) multiplied by the operating angle (θ) or an rpm-angle value (N·θ) increases. In general, as a guide, the upper limit of the rpm-angle value (N·θ) is such that N·θ>20,000 to 22,000. The Lobro-type constant velocity universal joint is now facing the task of increasing the limit rpm-angle value (N·θ) as much as possible.
Typically, the ball 15 employed in the Lobro-type constant velocity universal joint is tempered at about 180° C. after having been quenched. However, above the limit rpm-angle value (N·θ), the austenite retained in the components of the ball is varied due to the influence of temperature, thereby resulting in an increase in size. This is accompanied by an increase in pre-load, which in turn causes the temperature of the constant velocity universal joint to be further increased or results in a significant peak temperature. Some example in the past shows that the ball 15 having a normal HRC hardness greater than or equal to 60 is degraded to have an HRC of nearly 40 after operation due to the occurrence of the peak temperature.
As means for addressing such a problem, a ball 15 has been suggested which is subjected to a high-temperature size stabilizing treatment at a higher tempering temperature (disclosed in Japanese Patent Laid-Open Publication No. 2000-74082). However, an increase in tempering temperature has caused another problem to occur which the ball 15 inevitably have a lower hardness, thereby impairing the durability of the constant velocity universal joint.
There is another problem with the conventional Lobro-type constant velocity universal joint. That is, since the inner and outer rings 13, 14 have track grooves with a small curvature and a small vertex clearance at the bottom of the groove, a thermal deformation resulting from an increase in temperature of the constant velocity universal joint would cause the ball 15 to be brought into contact with the bottom of the groove. This would lead to a degradation in presence of lubricant, thereby causing a further increase in temperature. The contact of the ball 15 with the bottom would prevent a smooth operation of the constant velocity universal joint.